Tuna Nicoise Sandwich

Name: Tuna Nicoise Sandwich
Ingredients: 1/2 white onion thinly sliced; 1/2 red onion thinly sliced; 1/4 cup plus 4 teaspoons extra-virgin olive oil; 2 tablespoons red wine vinegar; Coarse salt and freshly ground pepper; 2 jars (7 ounces each) best-quality tuna packed in olive oil; 1 tablespoon fresh lemon juice plus lemon wedges, for serving; 4 small ciabatta rolls split horizontally; 2 heads Bibb lettuce leaves split horizontally; 1 1/2 cups nicoise olives pitted; 8 hard-boiled eggs peeled and thinly sliced